Disable mount(8) canonical paths in do_mount()

By default the mount(8) command, as invoked by 'zfs mount', will try
to resolve any path parameter in its canonical form: this could lead
to mount failures when the cwd contains a symlink having the same name
of the dataset being mounted.

Fix this by explicitly disabling mount(8) path canonicalization.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #1791 
Closes #6429 
Closes #6437

etc/systemd/system/zfs-mount.service.in

@@ -11,7 +11,6 @@ Before=local-fs.target
 Type=oneshot
 RemainAfterExit=yes
 ExecStart=@sbindir@/zfs mount -a
-WorkingDirectory=-/sbin/
 
 [Install]
 WantedBy=zfs-share.service

lib/libzfs/libzfs_mount.c

@@ -344,8 +344,9 @@ zfs_is_mountable(zfs_handle_t *zhp, char *buf, size_t buflen,
 static int
 do_mount(const char *src, const char *mntpt, char *opts)
 {
-	char *argv[8] = {
+	char *argv[9] = {
 	    "/bin/mount",
+	    "--no-canonicalize",
 	    "-t", MNTTYPE_ZFS,
 	    "-o", opts,
 	    (char *)src,